Don't build on Ubuntu 10.10

Its C++ compiler is too old.

http://hydra.nixos.org/build/12385722
Fix compilation error on some versions of GCC

src/libexpr/primops.cc:42:8: error: looser throw specifier for 'virtual nix::InvalidPathError::~InvalidPathError()'
src/libexpr/nixexpr.hh:12:1: error:   overriding 'virtual nix::EvalError::~EvalError() noexcept (true)'

http://hydra.nixos.org/build/12385750